While they observed no increase in children enrolled in special
ed classes, they observed the rise in children diagnosed with autism coincided
with an equal decrease in students diagnosed with other intellectual
disabilities.

Lead researcher Santhosh Girirajan, an assistant professor of
biochemistry and molecular biology at Penn State, said researchers have been
struggling to sort developmental disorders into categories based on observable
clinical features.

However, he said, it gets even more complicated with autism
because every individual can show a different combination of features.

“The tricky part is how to deal with individuals who have
multiple diagnoses because the set of features that define autism is commonly
found in individuals with other cognitive or neurological deficits,” he said in
a press release.

Autism Can Mean A Lot of Different Things

Autism — medically known as autism spectrum disorder — envelopes
a range of symptoms that impact a child’s social, communication, and behavioral
abilities.

An autism diagnosis now means a child could have autistic
disorder, pervasive developmental disorder not otherwise specified (PDD-NOS),
or Asperger disorder, the CDC states.

As many of these symptoms overlap, such as emotional
disturbances, it’s often difficult to pinpoint a single condition. The CDC
estimates autism co-occurs with other chromosomal, developmental, genetic,
neurologic, or psychiatric conditions up to 83 percent of the time.

Reclassifying Percentage Is High

Using the data from the IDEA program, researchers noticed a
three-fold increase of cases of autism from 2000 to 2010, but 65 percent of
that increase could be caused by reclassification. The diagnostic
reclassification of individuals from the category of intellectual disability to
the category of autism accounts for a large proportion of the change, which
varied depending on the age of the children.

The change varied depending on the age of the children,
researchers noted. They estimate that 59 percent of the increase in autism
diagnosis among 8-year-olds was due to reclassification. The same was true for
as many as 97 percent of 15-year-olds.

Giriajan says because autism and other intellectual
disabilities occur together so frequently they’re likely due to shared genetic
factors in many neurodevelopmental disorders.

While diagnostic tools “lose specificity” when applied to
people with different genetic disorders, future studies of autism prevalence
should take into account detailed genetic analysis, Girirajan says.

“Because features of neurodevelopmental disorders co-occur at
such a high rate and there is so much individual variation in autism, diagnosis
is greatly complicated, which affects the perceived prevalence of autism and
related disorders,” he said. “Every patient is different and must be treated as
such.”

The research was funded by the Penn State Huck Institutes of
the Life Sciences and the Penn State Department of Biochemistry and Molecular
Biology.